Prannoy Suraneni

Setting Time Measurement Using Ultrasonic Wave Reflection

Ultrasonic shear wave reflection was used to investigate setting times of cement pastes by measuring the reflection coefficient at the interface between hydrating cement pastes of varying water-to-cement ratio and an ultrasonic buffer material. Several different buffer materials were employed, and the choice of buffer was seen to strongly affect measurement sensitivity; high impact polystyrene showed the highest sensitivity to setting processes because it had the lowest acoustic impedance value. The results show that ultrasonic shear-wave reflection can be used successfully to monitor early setting processes of cement paste with good sensitivity when such a very low impedance buffer is employed. Criteria are proposed to define set times, and the resulting initial and final set times agreed broadly with those determined using the standard penetration resistance test.

Monitoring Setting of Geopolymers

The setting behavior of geopolymer pastes as a function of time was studied using two methods: penetration resistance and ultrasonic shear wave reflection. Several starting materials were included—metakaolin, Class C and Class F fly ashes, and slag—and chemical parameters known to affect set were varied. The geopolymers showed a wide range of set times compared to a reference Portland cement paste: some were much more rapid, some were similar, and some were much slower. Although most geopolymers formed gels that were both solid and had measurable strength, some, initially, formed a soft gel that had no measurable strength. Therefore, to fully characterize setting behavior, it is necessary to use both types of tests. It was seen that setting behavior was sensitive to chemical parameters, with setting delayed somewhat with increasing silica/alumina and increasing water/alkali, and accelerated substantially with calcium hydroxide substitution.

Calcium Oxychloride Formation Potential in Cementitious Pastes Exposed to Blends of Deicing Salt

Chloride-based deicing salt solutions can react with the calcium hydroxide in the cementitious matrix, leading to the formation of calcium oxychloride. Calcium oxychloride formation has been implicated in the premature deterioration of pavement joints and concrete flatwork across cold regions in North America. This study examines the formation of calcium oxychloride in the presence of blends of different chloride-based deicing salts (sodium and calcium chloride). This evaluation was performed using several plain cementitious pastes and pastes with fly ash, slag, and silica fume used as supplementary cementitious materials. Fly ash and slag were used at 20% replacement levels and the silica fume was used at 3 and 6% replacement levels. Thermogravimetric analysis was used to quantify the amount of calcium hydroxide, and low-temperature differential scanning calorimetry was used to quantify the amount of calcium oxychloride formed. When the salt blends consist of less than 20% of calcium chloride by mass, the amount of calcium oxychloride that forms is low and does not depend on the calcium hydroxide content in the pastes. When the salt blends consist of more than 20% of calcium chloride by mass, the amount of calcium oxychloride that forms depends on the calcium hydroxide content in the paste and increases with calcium hydroxide content. This suggests two strategies to mitigate the amount of calcium oxychloride that is formed: reduction in the amount of calcium hydroxide in the pastes through use of supplementary cementitious materials, and the use of deicing salt blends that include lower amounts of calcium chloride. A model is developed to estimate the amount of calcium oxychloride formed in mixtures, given the calcium hydroxide and calcium chloride contents.

Use of Fly Ash to Minimize Deicing Salt Damage in Concrete Pavements

Premature damage has been observed at the joints in numerous concrete pavements where calcium chloride and magnesium chloride deicing salts have been used. This damage results from a reaction between the deicing salt and the calcium hydroxide (CH) in the hydrated cement paste. This reaction leads to the formation of an expansive product known as calcium oxychloride (CAOXY). The use of supplementary cementitious materials as a replacement for cement has been proposed to reduce the CH that is available in the mixture to react with the deicing salts. Reducing the CH can reduce the amount of CAOXY that forms. In this study, mixtures representative of paving concrete were made with cements and fly ashes from across the country. CH amounts were determined by using thermogravimetric analysis, and CAOXY amounts were determined by using low-temperature differential scanning calorimetry. Various replacement levels of fly ash were used to demonstrate that the main parameter that influences the amounts of CH and CAOXY that form is the replacement level of fly ash. This paper proposes that a prescriptive specification requiring 35% cement replacement by volume with fly ash would reduce the damage caused by CAOXY formation and further proposes a performance specification to limit the CAOXY formation to below 15 g/100 g paste.

The Influence of Calcium Chloride on Flexural Strength of Cement-Based Materials

Calcium chloride (CaCl2), which is commonly used as a deicing salt, can react with calcium hydroxide (Ca(OH)2) in cement-based materials to form calcium oxychloride. This reaction causes damage that typically manifests itself as flaking of concrete pavements at the joints and leads to expensive repairs and a reduction of the service life. In this paper, cement pastes with different fly ash replacement levels were prepared to provide pastes with differing amounts of Ca(OH)2. Thermogravimetric analysis was used to quantify the Ca(OH)2 content in these pastes. Low-temperature differential scanning calorimetry (LT-DSC) was used to quantify the amount of calcium oxychloride formed when these pastes were exposed to CaCl2 solutions. The reduction in the flexural strength of these pastes saturated with different CaCl2 solutions was also measured. As the concentration of CaCl2 increases, the reduction in flexural strength increases. There is a lower flexural strength reduction in pastes with fly ash, because these pastes have lower Ca(OH)2 and form lower amounts of calcium oxychloride. The strength reduction is directly correlated to the amount of formed calcium oxychloride.

Micro-reactors to Study the Reaction of Slag in Alkaline Media

A micro-reactor approach to study the hydration of cementitious materials has been developed in our laboratory. The method involves milling gaps that are a few microns in dimension in grains using a Focused Ion Beam. These gaps are then filled with solution, leading to dissolution, nucleation, and growth in the gaps. Hydration is stopped at selected time intervals, and a Scanning Electron Microscope is used to image the gaps. Information is obtained about dissolution-growth kinetics and hydrate morphology. Using this technique, we were able to obtain significant insights into the factors influencing early age hydration of tricalcium silicate and alite, and the effect of selected chemical admixtures on the same. In this study, we present the use of micro-reactors to study the alkaline activation of slag. The effects of solution pH and of the nature of the alkaline solution on the alkaline activation are discussed. Additionally, the dissolution of slag is studied, and we show that it is strongly affected by the presence of calcium and aluminum in solution. Results are compared and contrasted with those obtained with tricalcium silicate and alite. Originality The micro-reactor approach is probably the only method that offers the possibility of obtaining information about both dissolution and growth processes. We present ways in which factors affecting dissolution and growth may be understood, something that is very difficult or impossible using other techniques. Applying the micro-reactor technique to alkaline activation offers a way to better understand the reaction and the factors influencing it. Additionally, although we only present results with slag here, this technique may be used to study geopolymerization of several precursors with various solution compositions.

The Influence of Cellulose Nanocrystals on the Hydration and Flexural Strength of Portland Cement Pastes

Recent research has shown that cellulose nanocrystals (CNCs) can be used at low dosage levels (approximately 0.2% by volume of cement) to increase the extent of hydration and to improve the flexural strength of cement pastes. However, the previous work was based on using a CNC made from a single source material and processing technique and was performed using only Type V cement. This work examines the influence of various raw material sources and processing techniques used to make the CNCs. In total, nine different CNCs were investigated with pastes made using Type I/II and Type V cements. Isothermal calorimetry (IC), thermogravimetric analysis (TGA) and ball-on-three-ball (B3B) flexural strength testing were used to quantify the performance of CNC-cement composites. IC and TGA results showed that CNCs increased the degree of hydration in all systems. IC results showed that the increase in total heat release was greater in the Type V than in the Type I/II cement paste systems. B3B flexural testing indicated an increase in flexural strength of up to 20% with both Type I/II and Type V systems. These results also showed that the performance of CNC-cement composites can be affected by the source and manufacturing process used to make the CNC.

Measuring Volume Change Caused by Calcium Oxychloride Phase Transformation in a Ca(OH)2-CaCl2-H2O System

Calcium oxychloride has been reported to form in cementitious materials when calcium chloride (CaCl2) solutions react with calcium hydroxide [Ca(OH)2]. In this study, Ca(OH)2 is mixed with CaCl2 solutions with concentrations of 5 %, 10 %, 15 %, 20 %, 25 %, and 30 % by weight, using a 1:1 M ratio of Ca(OH)2 to CaCl2. The Ca(OH)2-CaCl2 solution mixtures are subject to a cooling and heating cycle. Volume change is measured to quantify the phase transformation associated with calcium oxychloride. Low-temperature differential scanning calorimetry (LT-DSC) is used to construct a phase isopleth, which is used to quantify the phase transformation associated with calcium oxychloride. Hysteresis is observed in the volume-change measurement during the cooling–heating cycle. In a temperature range of 50°C to 0°C, the formation of calcium oxychloride is complete for the 20 %, 25 %, and 30 % CaCl2 solutions. The liquidus temperatures at which calcium oxychloride is expected to form from LT-DSC during heating match those from the volume-change measurements.

Performance of Concrete Pavement in the Presence of Deicing Salts and Deicing Salt Cocktails

• Some concrete pavements have shown premature deterioration at the joints. It has been proposed that this can be attributed to two primary factors: increased fl uid saturation and a chemical reaction that occurs between deicing salts and the cement matrix. • A test method was developed/formalized that uses a low temperature diff erential scanning calorimeter (LTDSC) test method to quantify the chemical reaction that occurs between the cementitious matrix and the deicing salt to form calcium oxychloride. • It is proposed that the LTDSC test be used to qualify the potential for calcium oxychloride formation in a cementitious matrix. Currently two primary JOINT TRANSPORTATION RESEARCH PROGRAM

Set Time Measurements of Self-Compacting Pastes and Concretes Using Ultrasonic Wave Reflection

The ultrasonic shear-wave reflection coefficient at the interface between a hydrating material and a buffer material was monitored immediately after mixing as a function of time to study setting behavior of self-compacting pastes and concretes. High-impact polystyrene was used as the buffer material because it is very sensitive to early changes. Self-compacting pastes were produced using fly ash and superplasticizer. Initial and final set times were measured and compared with those obtained from the standard penetration resistance test. Both methods showed reproducible results with self-compacting pastes and were able to distinguish fairly small changes in composition. A good correlation was found between set times obtained from the two methods. The self-compacting pastes were seen to have a substantially delayed set as compared with plain pastes. In addition, self-compacting concretes were made and their set times were measured using ultrasonic wave reflection. The method was seen to be quite reproducible for concrete, and concrete set times matched those of the corresponding pastes reasonably well.